Magnesium is an important plant nutrient component for those requiring for life chlorophyll. It also promotes root withdrawal of the plant nutrient phosphorus.
On account of the fact of the agronomo/culturally efficient utilization of tillable ground, considerable amounts of magnesium are removed, and since magnesium loss further takes place through washing out or fixation in the ground, a sufficient provision of the agricultural culture plants with magnesium is assured in many cases only by means of applying magnesium-containing fertilizer.
Previously, the amounts of magnesium hereby removed or threin the earth fixed are replaced by addition of magnesium-containing mineral fertilizers such as for example potassium fertilizer salt or magnesium-containing multiple nutrient fertilizer. For ameliorating acute magnesium deficiency in the soil, previously kieserite is frequently also employed.
A good provision of magnesium as well as potassium and phosphorus for the plants can be guaranteed by fertilizing with potassium magnesium phosphate. This compound contains no "ballast" material and can also be worked up into multi-nutrient fertilizer. Such multi-nutrient fertilizer possesses a high total nutrient content. They are described for example in DE-PS Nos. 11 52 210 and 15 92 903 which are hereby incorporated by reference.
Several production methods for potassium magnesium phosphate are already known, according to which the compound can be obtained water-free as monohydrate or as hexahydrate. Potassium hydrogen phosphate is reacted with magnesium oxide respectively carbonate in amounts of water bound thereby as water of crystallization.
According to a further production technique the potassium magnesium phosphate hexahydrate was obtained by reaction of Engel's salts (MgCo.sub.3.KHCO.sub.3.4H.sub.2 O) with phosphoric acid. See, e.g. Gmelins Handbuch der Anorganischem Chemie, 8. Auflage, System-Nr. 27 (B), Liefuring 4 (1939), pages 465-466. For this technique the potassium salt must first however be prepared through the additional technique expenditure of potassium chloride.
According to the teachings of FR-PS No. 722378, products are formed by kneading calcium dihydrogen phosphate with potassium sulfate and magnesium oxide containing potassium magnesium phosphate.
Similar products were described in DE-AS No. 17 67329, produced by working in of potassium sulfate into an acid, calcium and magnesium phosphate containing base mass. Such products are however contaminated with calcium sulfate.
In DE-PS No. 619 397 it is therefore recommended to heat potassium chloride with magnesium chloride and phosphoric acid until chlorhydrogen no longer escapes and the formation of the potassium magnesium phosphate is terminated. The technical accoutrements for the performance of this process must be composed however from corrosion-resistant material and, moreover, the exhaust gas must be removed under additional technical cost without injury to the environment.
From JP-AS No. 72 17 694, BE-PS No. 64 81 111 and GP-PS No. 969 419 it is known to precipitate potassium magnesium phosphate from sea water in contaminated form. A process for producing potassium magnesium phosphate by conversion of magnesium oxide and potassium hydroxide is described in DE-OS No. 19 24 284 with phosphoric acid.
Further procedures are known from DE-PS No. 12 65 726, U.S. Pat. No. 3,615,186 and BE-PS No. 740307. Whereby potassium magnesium phosphate is produced in the presence of organic amine through reaction from potassium and magnesium chloride respectively sulfate with phosphoric acid. The use of amine makes necessary however an additional technical purification of the potassium magnesium phosphate, which can only be realized through further expense.
DE-PS No. 28 31 672 suggests a process for producing potassium magnesium phosphate by reacting from acid, phosphate ion containing aqueous solution and potassium chloride and basic reacting magnesium compounds. According to an other as yet unpublished suggestion, potassium magnesium phosphate can be produced from sulfate mother liquor, whereby aside from potassium sulfate also potassium chloride is employed as potassium component.
With both techniques the mother liquor must be set out of the potassium magnesium phosphate crystallization and can not again be employed in the process.